In telecommunications systems, the end portions of the link between subscriber premises are typically served by multipair cable. This form of communication medium typically comprises numerous conductors arranged as twisted pairs which are encompassed by a protective sheath. The sheath generally includes a continuous metallic layer surrounding the core which affords electrical and mechanical protection to the pairs and, additionally, defines the electrical transmission properties of the pairs. The integrity of the sheath may be broken in a number of ways. For instance, an undetected defect may arise during manufacture or, more commonly, during usage. Oftentimes, a buried or underground cable is subject to lightning strokes which may introduce pinholes in the sheathing. If a defectively sheathed cable is utilized in an environment wherein water ingress to the core may occur, the problem of locating other faults affecting the pairs is compounded. For example, a pair may be rendered defective at a point geographically removed from the point of water contamination; the pair defects may include opens, shorts, grounds and so forth.
Regarding opens, testing techniques presently utilized by cable maintenance and repair personnel for locating opens in cables measure either (i) capacitance-to-ground of the open conductor accessed at a convenient test point with the other conductor of the pair grounded at the test point, or (ii) the mutual capacitance of the pair at the point of access. If the pair being measured is completely dry, such a measurement yields the distance to the open conductor or conductors with an accuracy of a few percent. However, if the cable is wet, error will be introduced and it may range up to a few hundred percent for a completely saturated section. Since it is not usually known or even suspected that water may be present in cable sections, huge measurement errors might be experienced with a concomitant increase in the time needed to locate conductor breaks.
Besides the conventional capacitance measurements to determine conductor lengths, other pertinent art relates to testing for the presence of water in cable for rehabilitation or replacement. Thus, the sole purpose of this type of testing is that of determining the need for replacing certain sections of cable. This is in contrast to the general problem addressed herein of determining the distance to an open not resulting from water migration and located at a point remote to the water-filled section or sections.
The test sequence utilized to determine the presence of water for rehabilitation purposes requires that two measurements be made on a pair, namely, (i) an open-circuited mutual capacitance measurement to yield a so-called "capacitive length," and (ii) a short-circuited DC resistance measurement to yield a so-called "resistive length." The ratio of capacitive length to resistive length is computed, and this ratio is plotted on a preexisting graph which depicts "ratio" versus "% water in cable." Thus, the procedure requires a compilation of a set of reference graphs based on cable types as well as a set of two-ended measurements. Such a technique proves cumbersome and does not provide sufficient information to locate an open.